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Sept. 6, 2012 -- Each year, a very small number of Americans get plague, the disease that killed millions in the Middle Ages.

Your odds of getting plague are vanishingly small. But it does happen. That's because plague bacteria have found a permanent home among rodents.

Are there different kinds of plague?

Plague is a disease caused by Yersinia pestis bacteria. The disease has three forms:

Bubonic plague comes from the bites of an infected flea or from contact with an infected animal.

Pneumonic plague comes from droplets coughed into the air by a person or animal whose lungs are infected with plague bacteria.

Septicemic plague comes from plague bacteria that get into the bloodstream. Bubonic plague sometimes progresses to septicemic plague.

How common is plague in the U.S.?

On average, there are seven cases of plague in the U.S. each year. In the last few decades, there have been as few as one and as many as 17 cases in a given year.

Where in the U.S. is plague most common?

Plague-carrying rats from Asia escaped ships docked in U.S. ports in 1900. Fleas from the rats spread the disease to squirrels, mice, prairie dogs, and other rodents.

The last U.S. city to have a plague outbreak was Los Angeles, in 1924-1925. Since then, human cases have popped up in rural areas scattered across Western states.

Most cases are in two regions:

Northern New Mexico, northern Arizona, and southern Colorado

California, southern Oregon, and western Nevada near the California border.

How do people get plague?

Most human cases come from flea bites. The fleas come from pets, animals carried into the house by a pet, or a dead animal.

For example, in 2012, a 7-year-old girl on a picnic in southwest Colorado came upon a dead squirrel. She put her sweatshirt down near the animal and asked for permission to bury it. When her parents said "No," she tied the sweatshirt around her waist. Later she had flea bites around her waist. (Thanks to quick-thinking doctors, the girl recovered).

People can also be infected from the blood of a dead animal, usually while skinning rabbits or other game.

And rarely, people can get the pneumonic form of plague from droplets coughed or sneezed by a person or animal.

What are the symptoms of plague?

Symptoms of the bubonic form of plague usually start two to six days after a flea bite or contact with an infected animal. Symptoms include high fever, headache, chills, weakness, and muscle aches. Usually one or more lymph nodes in the groin, armpit, or neck become swollen and extremely painful. These swollen lymph nodes are called buboes.

When plague bacteria get directly into the blood, they cause the septicemic form of plague. In this case, the lymph nodes do not swell. Symptoms are fever and severe flu-like symptoms. Sometimes untreated bubonic plague becomes septicemic plague.

When plague bacteria infect the lung, pneumonic plague results. Pneumonic plague can spread from person to person through airborne droplets. The first signs of pneumonic plague are fever, headache, weakness, and rapidly worsening pneumonia with shortness of breath, chest pain, and sometimes bloody or watery sputum.

Untreated bubonic plague is fatal more than half the time. Untreated pneumonic plague is almost always fatal.

What is the treatment for plague?

Plague is a dangerous disease, but it's curable with early antibiotic treatment. The sooner treatment begins, the better the odds of full recovery.